This invention relates to a frequency-hopped LAN radio system which comprises stations and access points (APs). For example, a station may comprise a notebook PC (personal computer) and a radio LAN adapter. More particularly, this invention relates to a method of establishing, on the station, synchronous communication with a suitable access point selected among the access points.
Wireless LAN technologies have rapidly grown in the art of computer networks. They provide user connectivity without being tethered by wired networks. One of the systems using the wireless LAN technologies is a LAN radio system in which radio frequencies are used for connection. Such LAN radio system employs a spread-spectrum modulation to enable multiple users to access to the network.
Frequency hopping (FH) is one form of the spread-spectrum modulation, in which a center frequency of a conventional carrier is altered many times a second in accordance with a pseudorandom or pseudonoise (PN) list of channels. Such list of the channels is referred to as a hopping pattern, in which the channels having the center frequencies to be altered are distinguished from each other by hopped frequency indexes corresponding thereto. The amount of time, for which a signal is present on any single channel, is called a dwell time.
A LAN radio system using the frequency hopping technology, called a frequency-hopped LAN radio system, comprises the access points working asynchronously for each other. Each of the access points has the hopping pattern and the dwell time determined for the hopping pattern, and covers a zone assigned for each access point to provide connection service. Roughly speaking, the access points transmit control frames that include information for synchronizing with the access points, while the station uses control frames to obtain the synchronization with one of the access points, in the frequency-hopped LAN radIo system. These are described, for example, in the IEEE Std 802.11-1997 xe2x80x9cWireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specificationxe2x80x9d, which is incorporated herein by reference.
To realize seamless communication during moving of the station between the zones, the frequency-hopped LAN radio system makes the zones of the access points overlap each other. In such system, the station needs to select one among the access points and to synchronize with the selected access point, when the station newly participates in the system or when the station transfers from one zone to another zone.
This shows that each station must select the access point and establish synchronization with the selected access point. However, it should be considered that the access points perform asynchronously to one another. As a result, it is difficult to select a desired one of the access points and to establish the synchronization with the desired access point, even if the control frames are used.
For example, the selection together with synchronization is disclosed in JP-A No. 8-125582. However, the disclosed selection method requires each of the stations to change the frequencies of control channel and data channel, in turn, and is too complex. Besides, the complex selection loses many periods of times and brings about inefficient communication.
Another selection known to the inventors is to start the selection and the synchronization at a time instant when the station receives first one of the control frames. This selection is simple to be realized, and however, the selected access point depends on the first received control frame. Therefore, the actually selected access point is not always the suitable one among the access points.
This invention therefore provides the method of establishing, on the station, synchronous communication with a suitable access point selected among the access points.
According to one aspect of the present invention, a following method of establishing, in a station, synchronous communication with an access point selected among a plurality of access points in a frequency-hopped radio LAN system is obtained. Herein, each of the access points is assigned with a hopping pattern of hopping frequencies and a dwell time determined for the hopping pattern. The access points transmit frames which include control frames.
In establishing method according to this aspect of the invention, the station selects a communication frequency of the station among the hopping frequencies. And then, the station receives, as reception frames, the frames which are transmitted from the access points through the selected communication frequency. The station extracts the control frames from the reception frames, and monitors the extracted control frames during a monitoring time interval. Herein, the monitoring time interval is predetermined in consideration of the dwell times assigned for the access points. After that, the station determines the selected access point and the hopping pattern assigned for the selected access point, on the basis of the results of the monitoring. And also, the station synchronizes itself with the selected access point on the basis of information included in the extracted control frames, together with the dwell time assigned for the selected access point.
More in detail, each of the hopping pattern has the hopping frequencies which are hopped at every dwell time corresponding to one of the access points and which are distinguished from each other by hopped frequency indexes, and the station may attain the synchronization as the followings.
The station decides a time instant at which the station will be synchronized with the selected access point, in consideration of ability of the station. And then, the station calculates the hopped frequency index corresponding to the decided time instant. The station uses the calculated hopped frequency index and the hopping pattern of the selected access point, to predict, as a predicted frequency, a usable one of the hopping frequencies that will be used for the selected access point at the decided time instant. The station obtains a remnant time from the decided time instant to an end time instant at which the dwell time of the predicted frequency will be finished. And thereby, the station adjusts the communication frequency of the station to the predicted frequency at the decided time instant, so as to communicate with the selected access point on the predicted frequency during the remnant time.
The above-mentioned method may be embodied in the form of a radio LAN adapter, which is adapted to be installed in the station and enables the station to the above operations.